In recent years, techniques for discriminating forgery of a valuable printed member such as a bank note and securities have been required and various techniques for checking authenticity of the printed member have been proposed. As one of the techniques, a technique has been adopted in which a predetermined magnetic pattern is formed on a paper with magnetic ink and presence/absence of the magnetic pattern or its formed position is detected from the medium on which the magnetic pattern has been formed (see, for example, Japanese Patent No. 3,799,448 and Japanese Patent No. 3,814,692).
The recognition method is generally classified into three methods. In other words, the first method is a method in which presence/absence of magnetic ink and its formation position is detected through a differential transformer type sensor by utilizing a difference of permeability of a medium (magnetic ink). The second method is a method in which magnetic ink is magnetized and then its leakage flux is measured by an MR sensor, an MI sensor or a magnetic head. This method measures residual magnetic flux density of the magnetic ink. The third method is a method in which, when magnetic ink is to be magnetized with a magnet, a variation of magnetic flux of the magnet is measured with an MR sensor and this method measures permeability of the magnetic ink.
However, in paper moneys or the like, there may be cases that kinds of used magnetic inks are different for each country or for each paper money and, in this state, it is difficult that all paper moneys are discriminated by one device. Generally, magnetic ink is roughly classified into magnetic ink containing hard material and magnetic ink containing soft material. The hard material is, like magnetic material used for a magnet, magnetic material whose hysteresis is larger and residual magnetic flux density is higher and which is easily magnetized when magnetic field is applied from the outside. On the other hand, the soft material is, like core material for a motor or a magnetic head, magnetic material whose hysteresis is smaller and residual magnetic flux density is lower and which is not easily magnetized. Therefore, in the first method and the third method as described above, in magnetic types on a weak side, an output having an extremely low level is obtained and thus it is difficult to determine the type of magnetic ink. Further, even in the magnetic ink containing hard material on a strong side, since an output level is varied due to shading, an output is small in a case of a magnetic ink with a low concentration and thus, in this case, it is difficult to determine whether it is the hard material or the soft material. Therefore, in order to discriminate authenticity of a paper money or its kind with a high degree of accuracy, after it has been judged whether the magnetic ink contains hard material or soft material, discrimination is required to perform by a method suitable to the result of the judgment and thus it is difficult that all kinds of paper moneys are discriminated by one device. In the devices disclosed in the above-mentioned two Patent References, plural kinds of magnetic patterns whose residual magnetic flux densities are different from each other are detected but, even in these devices, substantially, only the residual magnetic flux density of the magnetic ink is measured.
Further, it is conceivable that a detection device measuring permeability is disposed on an upstream side of a feeding path for a paper money and another detection device measuring a residual magnetic flux density is disposed on a downstream side of the feeding path for the paper money. However, in this structure, respective errors of two detection devices affect the recognition and thus accurate discrimination is difficult. In addition, when two detection devices are provided, cost is increased by themselves and, since two mechanically stable traveling portions are required, a complicated mechanism is required and thus cost is further increased and it is not practical.
In view of the problems described above, at least an embodiment of the present invention is provides a magnetic pattern detection device which is capable of surely detecting presence/absence of the magnetic pattern and types of magnetic inks on various media with a simple structure.